The present invention relates to a process for manufacturing, moulding and curing tires for vehicle heels. More particularly, the present invention relates to a process for manufacturing, moulding and curing tires for vehicle wheels in which provision is made for admission of a fluid under pressure directly in contact with the green tire, i.e. in the absence of a vulcanization bladder.
In a cycle for vehicle wheel tire production it is provided that, subsequently to a manufacturing process in which the different tire components are made and assembled, a moulding and curing process should be carried out for the purpose of giving the green tire a desired geometrical conformation and a particular tread pattern, and subsequently stabilizing this conformation by vulcanization of the elastomer material forming the tire itself.
For this purpose, the green tire is enclosed in a moulding cavity defined within a vulcanization mould and the shape of which matches that of the outer surface of the tire to be obtained.
Within the scope of the present description, by xe2x80x9cgreen tirexe2x80x9d it is intended the product which is obtained after the manufacturing step in which the different tire components are assembled and the elastomer material has not yet been submitted to curing. By xe2x80x9couter surfacexe2x80x9d it is intended the tire surface which is in sight once said tire has been mounted on the wheel rim, whereas by xe2x80x9cinner surfacexe2x80x9d it is intended the tire surface which is in contact with the inflating fluid of the tire itself.
In one of the most widespread moulding methods it is provided that a vulcanization bladder made of rubber inflated with steam and/or another high-pressure heated fluid should be inserted into the green tire enclosed in the moulding cavity. In this manner the tire is conveniently pressed against the inner walls of the moulding cavity and strengthened into the geometric conformation imposed thereto, as a result of the cross-linking process to which the elastomer material forming the tire itself is submitted. Cross-linking is carried out by virtue of heat supply transferred from steam through the bladder, as well as from the mould walls.
Since the vulcanization bladder is a deformable element due to its own nature, its use involves several drawbacks such as geometric imperfections in the tire as a result of possible distortions undergone by the bladder itself, formation of flashes, in particular at the beads, due to undesired escaping of part of the elastomer material at the starting instants of the vulcanization process. In addition, thermal inertia typical of the material forming the vulcanization bladder represents a hindrance to heat transfer to the tire by the fluid admitted to the bladder itself.
Also known are curing methods in which steam or another heating fluid under pressure is directly admitted to the inside of the tire enclosed in the moulding cavity, in the absence of vulcanization bladder. Moulding methods without the aid of the above mentioned vulcanization bladders are described, e.g., in U.S. Pat. Nos. 4,236,883, 4,400,342, 5,127,811 and 5,597,429.
In accordance with other moulding methods it is provided that, in place of the inflatable vulcanization bladder, a rigid toroidal support having the same configuration as the inner surface of the tire to be obtained should be arranged within the tire itself. For instance, such a method is described in European Patent EP-242,840 in which a rigid toroidal support is employed for imposing the final shape and sizes to the tire enclosed in the mould. According to the above patent disclosure, the different thermal-expansion coefficient between the metallic toroidal support and the elastomer material of which the green tire is made is utilized for achieving an appropriate moulding pressure.
However, the process described in patent EP-242,840 presents many drawbacks in its practical accomplishment. First of all, a very precise and also very difficult control of the volumes of the material employed in manufacturing the tire is required. Besides, it is not possible that an appropriate radial and/or circumferential expansion should be imposed to the tire, for instance in order to achieve desired preloading effects in the reinforcing structures employed in making the same. Finally, obtaining a correct and efficient heat transmission to the tire inside is rather difficult.
In the co-pending European patent application No. 98830473.9, filed on Jul. 31, 1998 in the name of the same Applicant, a moulding and curing process for a tire with the aid of a rigid toroidal support is described, in which the necessary pressing of the raw elastomer material against the mould walls is carried out through the admission of a fluid under pressure into a diffusion interspace created between the inner surface of the green tire and the outer surface of the toroidal support. In this way, during the pressing step a radial expansion is imposed to the tire by effect of the pressurized-fluid admission, with an increase in the tire circumference included between 1.5% and 2.5%, measured at an equatorial plane of the tire itself. The pressurized-fluid admission is preferably carried out by means of feeding channels formed in the toroidal support and terminating at the outer surface of the latter. During the pressurized-fluid admission, the tire is sealingly engaged at its circumferential inner edges, between the walls of the moulding cavity and the outer surface of the toroidal support, so as to hermetically delimit the diffusion interspace at the circumferential inner edges of the tire itself. Advantageously, the admission of heat necessary for vulcanization takes place by admission of a heating fluid into the diffusion interspace. This heating fluid may form or at least be part of the same fluid under pressure utilized for carrying out the pressing step.
In the Applicant""s perception, processes without a vulcanization bladder as above described, in which a direct contact between the green tire and a fluid under pressure is provided, can cause many inconveniences due to permeation of the fluid itself into the structure of the tire not yet vulcanized. In fact the fluid permeation may, for example, either cause separations between the adjacent blend layers, or adversely interfere in the adhesion processes between the elastomer material and the metallic or textile reinforcing structures, or even promote corrosion phenomena in the metallic reinforcing materials.
In accordance with the present invention, these inconveniences are avoided by including a treatment step carried out on the inner surface of the green tire in the tire manufacturing process, so that said step prevents the permeation of the fluid under pressure into the tire structure.
Therefore, according to a first aspect, the present invention therefore relates to a process for manufacturing, moulding and curing tires for vehicle wheels, comprising the following steps:
manufacturing a green tire comprising a raw elastomer material;
closing the green tire into a moulding cavity defined inside of a vulcanization mould, said moulding cavity having walls the shape of which matches that of an outer surface of the tire when vulcanization has been completed;
admitting a fluid under pressure into the space defined by an inner surface of the tire in order to press the outer surface of the green tire against the walls of the moulding cavity;
supplying heat to the tire for causing the vulcanization of the raw elastomer material; characterized in that said process, before the stage of admitting fluid under pressure, further comprises a treatment stage to be carried out on the inner surface of the green tire suitable for preventing the permeation of the fluid under pressure into the inside of the tire itself.
In a preferred embodiment, said treatment carried out on the inner surface of the tire is performed by associating at least one layer of prevulcanized elastomer material with the inner surface of the green tire.
In the Applicant""s perception, the prevulcanization degree of the layer associated with the inner surface of the tire is suitable for obtaining a sufficient mechanical strength to diffusion and penetration of the fluid under pressure, and at the same time a high fatigue strength, in particular during the first moulding steps of the tire, in order to avoid formation of fissures and cracks.
In more detail, the stage of treating the inner surface of the green tire comprises the following steps: forming at least one layer of a raw elastomer material on an outer surface of a toroidal support the shape of which substantially matches that of the inner surface of the tire; manufacturing the green tire on the toroidal support carrying said layer of raw elastomer material; prevulcanizing the layer of raw elastomer material before introducing the green tire into the vulcanization mould.
Prevulcanization of the elastomer layer is conveniently carried out, at least partly, during manufacturing of the tire on the toroidal support.
In more detail, prevulcanization of the elastomer layer takes place by heat transfer to the elastomer layer itself through heating of the toroidal support.
Preferably, heating of the toroidal support is at least partly achieved by means of the toroidal support itself in a previous moulding and vulcanization cycle of a tire.
According to another preferred embodiment, formation of the layer of raw elastomer material is achieved by applying a vulcanizable liquid composition (in the following referred to as xe2x80x9cprimerxe2x80x9d) to the outer surface of the toroidal support.
Preferably, the primer comprises a polymeric base and a vulcanizing system suitable for being active during the green-tire manufacturing step.
According to a third form of embodiment of the present invention, the vulcanizing system which is present in the primer is complete and therefore active already at the moment the primer is applied to the toroidal support.
According to an alternative embodiment of the present invention, the vulcanizing system contained in the primer can be activated only when it is brought into contact with the raw elastomer material constituting the innermost layer of the tire. In this way an undesired prevulcanization of the primer before its application to the toroidal support is avoided. In other words, the primer is stable in time, so that it can be prepared and stored according to the production requirements, even a long time before its real application, this fact making the industrial accomplishment of the process easier.
The primer can be applied either in the form of a solution in a volatile organic solvent through spraying onto the outer surface of the toroidal drum and subsequent evaporation of the solvent, or by dipping the toroidal support into the primer, drawing the toroidal support out of said solution and evaporating the solvent.
According to a preferred form of embodiment, the primer is applied in the form of an aqueous emulsion by immersion or, preferably, by spraying and subsequent evaporation of the solvent (herein, water contained in the emulsion). In this manner, a prevulcanized layer of high compactness and uniformity can be obtained with no need to use any organic solvents which, as known, can create disposal and environmental pollution problems and therefore the application of the process becames inconvenient from an industrial point of view.
Preferably, after the primer has been applied and the solvent evaporated, a layer of elastomer material suitable for ensuring the retention of the tire inflating fluid (commonly referred to as xe2x80x9clinerxe2x80x9d) is applied to the layer of raw elastomer material thus obtained.
Preferably, the liner is previously manufactured in the form of a thin sheet and applied in order to cover the outer surface of the toroidal support.
Application of at least one intermediate layer having the same composition of the liner""s composition can be also provided in order to be interposed between primer and liner applications. Application of this intermediate layer is deemed to promote co-vulcanization between primer and liner, thereby greatly improving bonding of the prevulcanized layer to the overlying liner and therefore to the tire as a whole.
In the same manner as with the primer, application of the intermediate layer can take place by spraying or immersion, using either a solution or, preferably, an aqueous emulsion containing the liner blend.
After the treatment of the inner surface of the tire has been accomplished as above described, the subsequent manufacturing, moulding and curing steps are carried out according to the known art.